1. Field of the Invention
The present invention relates to a calibration system of a printing device and, more particularly, to a calibration system capable of executing efficient and high-quality calibration in an environment where a device is selectively used from a plurality of color measurement devices of varying accuracy during calibration.
2. Description of the Related Art
Generally speaking, an image formation device such as a printer performs image formation by ejecting ink or supplying toner onto a printing medium on the basis of image data represented by a density gradation value of each color. Further, the respective density gradation value of the image data undergoes image formation processing so that the densities (color value) of the image that is actually formed on the printing medium has the predetermined standard value (target value). Normally, image formation devices such as printers differ from one device unit to the next and the relationship (density characteristic) between the density gradation value and the color value that is actually output for the density gradation value is also different for each device unit. Therefore, color correction information (a color correction table, for example) that matches the concentration density of the device are established when the device is shipped from the factory and color correction processing based on the color correction information is performed for each of the density gradation values of the subject image data at the time of image formation.
However, variations in the environment and aging degradation of the respective parts occur as a result of using such an image formation device and variations arise in the state of the engine parts that actually perform the image formation on the printing medium. Accordingly, because the above density characteristics are also changed, the initial color correction information must be suitably adjusted in order to hold the output result at the target value.
As a result, calibration of the image formation device has conventionally been performed and one such method of calibration is a method that employs a patch sheet. Such a method outputs a patch sheet on which are printed a plurality of patch patterns produced by varying the gradation values of the respective image data for each color of the color materials (toner, ink or the like) used by the image formation device. Further, the density (color value) of the respective patch pattern on the patch sheet is read and measured by a color measurement device such as a color measurement mechanism and the color correction information (color correction table, for example) is updated in order to correct the differences between the target value estimated for the gradation value of the respective patch and the actual measurement value.
Because a plurality of patch patterns printed at color densities that correspond with gradation values of predetermined intervals are color-measured in such calibration, this does not mean that printing and color measurement of the patch patterns are performed for colors corresponding with all the gradation values constituting the subject of the color correction information (color correction table or the like). In other words, the color measurement results of the patch pattern are scattered data. As a result, color correction information (color correction table or the like) for all the gradation values is determined by performing processing to interpolate the gradation value intervals of the patch patterns at any stage from the scattered data thus obtained.
In this case, when the color measurement device for color-measuring the patch patterns is a device of low color measurement accuracy such as a scanner, errors contained in the individual measurement values are large and, therefore, the error also greatly affects the interpolation processing. Therefore, in order to keep the effect to a minimum, interpolation processing of all gradation values is performed by using a higher order polynomial with high interpolation accuracy when a color measurement device with low color measurement accuracy is used. However, because a multiplicity of referenced measurement values are required for this purpose, a patch pattern with a greater number of patches distributed thereon is required.
Thus, a patch pattern that permits highly accurate gradation-value interpolation, that is, which permits high-quality calibration, has different requirements depending on the relative merits of the color measurement accuracy of the color measurement device being used. Therefore, in an environment permitting the selection of a plurality of color measurement devices possessing the relative merits of color measurement accuracy on a network, the preferred patch patterns for raising the calibration accuracy of the printing device vary depending on the color measurement device. Hence, a method for changing the patch patterns that are printed in accordance with the type of color measurement device being used has been proposed. Such a method appears in Japanese Patent Application Laid Open No. 2001-232917, for example.
However, the conventional method above has been confronted by complications in that patch patterns that correspond with each of the color measurement devices must be prepared and a plurality of patch pattern data must be provided and managed. Further, suppose that, after printing patch patterns suited to a certain color measurement device on a patch sheet, the color measurement device that is being used is changed due to the inconvenience of the color measurement device or the selection of a more accurate color measurement device, there has been the problem that the printed patch sheet is then of no use. There has also been the possibility that, when a color measurement device is not connected during printing of the patch sheet and undecided, suitable patch patterns cannot be selected or a drop in the calibration accuracy is induced when a patch sheet with patch patterns for another color measurement device is used.
In addition, in the case of a color measurement device of low accuracy such as a scanner, there is a large number of patches as mentioned earlier, patch patterns are output over the whole surface of the printing medium, and the output area widens. Hence, the patches are susceptible to the effects of in-plane unevenness of printing and in-plane errors, which is unfavorable for high-quality calibration.